Intact subunit RNA (35S) was isolated from purified Rauscher murine leukemia virus. The viral RNA was translated in an in vitro protein synthesizing system which was extremely sensitive and one that had a very low endogenous background activity. The system is composed of a 10,000 xg supernatant from N.I.H. Swiss mouse tissue culture cells which was pretreated with a Ca ions-dependent nuclease to destroy endogenous mRNA. The withdrawal of the Ca ions rendered the nuclease inactive thus allowing the translation of added mRNAs such as globin and ovalbumin mRNAs. The in vitro protein product obtained by translating 35S Rauscher viral RNA was mainly a 68,000 dalton polypeptide precipitable with anti-serum to the major structural protein p30. Peptide mapping experiments showed it to be a polyprotein precursor to the four major viral structural proteins (p30, p15, p12, and p10). Proposed work for the coming year includes further characterization of a 200,000 dalton polypeptide as well as 60,000, 75,000 and 80,000 dalton polypeptides made in response to 35S viral RNA such as: determination of the relationship of these polypeptides to each other and to those precursor polypeptides found in the infected cell; investigation of the effects of the arginine analog, canavanine, on the inhibition of in vitro processing of cell-free synthesized viral specific polypeptides; search for "signal" peptide sequences in the in vitro translation products; search for reverse transcriptase and envelope protein sequences in the in vitro translation products.